1. Field of the Invention
The present invention relates to a bank angle detecting device for detecting, with a high accuracy, an angle of banking of a motorcycle during cornering of the latter and a headlight device for such motorcycle.
2. Description of the Related Art
During the travel of a motorcycle, the motorcycle generally makes the curve with its body being banked. However, considering that the headlight used in most motorcycles now available in the market is fixed in position to the motorcycle frame structure and, therefore, the region of illumination cast by the motorcycle headlight is correspondingly tilted as the motorcycle is banked, the amount of light distributed by the motorcycle headlight in an area inwardly of the direction of movement of the motorcycle, in which the motorcycle rider's eyes are focused, particularly during the cornering at night decreases during the cornering, with the rider's field of view forwardly of the direction of travel narrowed consequently.
More specifically, referring to FIG. 13 showing the forward field of view available to the motorcycle rider during the straight forward drive at night, the region of illumination A cast by the motorcycle headlight upon an area of the road surface forwardly of the motorcycle spreads in leftward and rightward directions of the motorcycle, which are parallel to the horizontal datum line H. However, as best shown in FIG. 14, when the motorcycle makes the curve in a leftward direction along a curved lane 80 as shown by the arrow-headed line P therein, the motorcycle makes a cornering while being tilted at a certain bank angle and, therefore, the region of illumination A cast similarly by the motorcycle headlight on an area of the road surface tilts downwardly leftward as compared with the region of illumination A exhibited during the straight forward run of the motorcycle. Therefore, a forward portion of the road surface inwardly of the direction of turn of the motorcycle (as indicated by the dotted circle B in FIG. 14), where the motorcycle rider's eyes are generally focused during the cornering encompasses less portion of the region of illumination A than that during the straight forward run, with the consequence that the rider's field of view forwardly of the direction of cornering is virtually narrowed.
In view of the foregoing, the headlight device capable of resolving the foregoing problems and inconveniences has been suggested, in which based on the bank angle of the motorcycle detected by a bank angle detecting unit, the lens and the light emitting element of the headlight can be pivoted or rotated about the optical axis of the headlight in a direction counter to the direction of tilting of the motorcycle. (In this connection, see, for example, the JP Laid-open Patent Publication No. 2004-155404 published Jun. 3, 2004.) With this suggested headlight device, as shown in FIG. 12, a forward portion of the road surface inwardly of the direction of turn of the motorcycle (as indicated by the dotted circle B in FIG. 12), where the motorcycle rider's eyes are generally focused during the cornering, can encompass more region of illumination A, with the rider's field of view forwardly of the direction of cornering increased consequently.
The bank angle detecting unit employed in the motorcycle headlight device such as discussed above is made up of a headlamp module for illuminating the forward area of the motorcycle, a light distribution adjusting mechanism for changing the region of illumination cast by the headlamp module, a bank angle detecting unit for detecting the bank angle of the motorcycle in reference to the rotatory angular velocity (yaw rate) about a vertical axis of the motorcycle, the motorcycle traveling velocity and an acceleration in a lateral direction of the motorcycle, and a light distribution control unit for controlling the light distribution adjusting mechanism based on the detected bank angle to change the region of illumination so that a far area inwardly of the direction of turn of the motorcycle during the cornering. As the bank angle detecting unit, a yaw rate sensor such as, for example, a gyro is employed and the bank angle δ can be obtained from the following equation (1), in which the traveling velocity is expressed by v, the acceleration of gravity is expressed by g and the yaw rate is expressed by ωy:δ=sin−1(v·ωy/g)  (1)
When the headlamp module is turned an angle, corresponding to the bank angle δ so determined by the equation (1) above, in a direction counter to the bank angle δ, the relatively large region of illumination by the headlamp module effective to increase the rider's field of view forwardly of the direction of cornering is obtained.
It has, however, been found that the bank angle δ that can be determined by the above discussed equation (1) is a static bank angle and, therefore, when affected by a pitching of the motorcycle body at the start of banking of the motorcycle or during acceleration resulting from opening or closure of an accelerator during banking, the change tends to become considerable. In other words, the bank angle δ changes at a short cycle. It may accordingly occur that if the headlamp module is rotated in a direction counter to the bank angle in accordance with the bank angle δ so varying as discussed above, the headlamp module itself will move at a correspondingly short cycle, resulting in unnatural or uncomfortable sensation to the rider.